I originally posted in the deVille forum, but haven't had any bites, so I figured I'd try here. I have had my 1990 deVille for two and a half years and it has been the best car until just recently. The car has been maintained very well (I have records dating back to 1991), and I have continued the practice. The car has 128,000 miles on it, of which 13,000 I have put on it.

The car has been getting harder and harder to start. The check engine light and service vehicle light come on immediately and then go away after a few minutes. Then driving down the road the speedometer stops working, both lights come back on and the car runs really rough, and I have to keep the gas pedal depressed to keep it from stalling.

I got the following readout when I had the Caddy show me the error codes on the digital display:
8.8.8
. .E
E12
E30
E52
E91
E98
. .F
F32
.7.0

One really interesting thing to note - after I displayed the error codes the car seemed to run a lot better for a few minutes. (I read them with the car turned on and off, just to see if there was any difference)

I did some research and found out what the codes mean. My questions are these:

1. What is the most likely root of all these problems? Should I go ahead and just replace the ECM and distributor?

2) How much is this going to cost me? I have a very very tight budget.

3) Is it worth fixing?

I love my car, but I'm not sure how much more I can spend to keep her running.

Thank you so much in advance for any help and or advice you can give me.

I just happen to be looking through my manual for a TPS issue with mine and I see you have some of the same codes. Not sure if it will be the same for yours but it points toward a number of things. Ignition module, pick up coil, loose wiring somewhere, fuel pressure low,vacuum leaks, I believe the Code 30 is ISC motor. The ECM also could be the culprit but really hard to tell until you scout out some of the other things first. There is a lot of detailed diagnostics for these codes. The 30 could be the idle switch as well which is part of the ISC motor.
Codes 51 thru 54 are usually ECM. Check all connections at distrubutor, ISC,TPS and ECM then you will know if the small stuff has helped any, then proceed to the big stuff.

When my 4.9L Seville had rough running problems with codes pointing to the distributor, I pulled the cap and rotor off to have a look. What I found was the plastic they used for the connectors to the ignition module was of a variety that wasn't resistent to heat. They baked in there and crumbled apart. You can't get these connectors easily but as long as the metal quick connects haven't corroded or melted you can slip each one onto each module pin. Have a schematic handy to match the color to the proper pin. If this happened to you, the rubber boot on the connector that seals up the hole where the wires come under the cap will have been destroyed as well. You will need to put a glob of RTV on this passage and screw the cap down. Allow it to dry for at least an hour before running it. If this area doesn't seal, you'll get moisture under the cap causing cross fire problems.

Check Fuel Pressure, this type of Cad has a ****ty gas tank and there is a plastic liner inside the tank, this plastic cradles the fuel pump assembly and when the plastic fails (pits, breaks off, becomes weak) gas that should be always present in that fuel pump's valley bleeds out to the tank and symptoms such as your's appear. Lack of fuel will will cause ECM failure and Idiot Lights go off. Check that first. If not then buy yourself a voltmeter and start checking OHM's at connectors vital to idle and ignition. Shouldn't cost much but a fuel pressure tool and a voltmeter if you don't already have such items. Also buy a bible (maintenance manual) for the Cad.